Op Amps for Everyone - The Repeater Builder's Technical ...

Additional Parameter Information
11.3.7.1 Input Capacitance
Input capacitance, C i , is measured between the input terminals with either input grounded.
C i is usually a few pF. In Figure 11–5, if V p is grounded, then C i = C d || C n .
Sometimes common-mode input capacitance, C ic is specified. In Figure 11–5, if V p is
shorted to V n , then C ic = C p || C n . C ic is the input capacitance a common mode source
would see referenced to ground.
11.3.7.2 Input Resistance
Input resistance, r i is the resistance between the input terminals with either input grounded.
In Figure 11–5, if V p is grounded, then r i = R d || R n . r i ranges from 10 7 Ω to 10 12 Ω, depending
on the type of input.
Sometimes common-mode input resistance, r ic , is specified. In Figure 11–5, if V p is
shorted to V n , then r ic = R p || R n . r ic is the input resistance a common mode source would
see referenced to ground.
11.3.8 Output Impedance
Different data sheets list the output impedance under two different conditions. Some data
sheets list closed-loop output impedance while others list open-loop output impedance,
both designated by Z o .
Z o is defined as the small signal impedance between the output terminal and ground. Data
sheet values run from 50 Ω to 200 Ω.
Common emitter (bipolar) and common source (CMOS) output stages used in rail-to-rail
output op amps have higher output impedance than emitter follower output stages.
Output impedance is a design issue when using rail-to-rail output op amps to drive heavy
loads. If the load is mainly resistive, the output impedance will limit how close to the rails
the output can go. If the load is capacitive, the extra phase shift will erode phase margin.
Figure 11–6 shows how output impedance affects the output signal assuming Z o is mostly
resistive.
Z O
V O A VD
R L
Z O
A VD
RL
V O
RL Z 0
A VD
CL
V O
V O
1
jff0 1
Where:
f O 1
2Z 0 C L
Resistive Load
Capacitive Load
Figure 11–6.Effect of Output Impedance
Some new audio op amps are designed to drive the load of a speaker or headphone directly.
They can be an economical method of obtaining very low output impedance.
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